1. Field of the Invention
The present invention relates to an image coding apparatus and method for compressing an image using wavelet transform and entropy coding according to, for example, JPEG-2000, an image decoding apparatus and method for decoding the compressed image, and a recording medium in which a program to allow a computer to execute an image coding process or an image decoding process is recorded.
2. Description of the Related Art
Conventional typical image compression systems include JPEG (Joint Photographic Experts Group) standardized by the ISO (International Standards Organization). JPEG uses discrete cosine transform (DCT). As is generally known, according to the DCT, favorable coded images and decoded images are provided when a bit-rate is relatively high. However, when the number of bits to be coded is less than a predetermined value, block distortion inherent in the DCT becomes noticeable. Thus, a subjective loss of image quality may be noticeably found.
On the other hand, there is a system in which an image is divided into a plurality of frequency bands using a filter called a filter bank composed of a high-pass filter and a low-pass filter and each frequency band component is then coded. In recent years, researches and studies of this system are increasing. Among them, wavelet transform coding is regarded as the most effective new technique that substitutes for the DCT because the wavelet transform coding does not have a disadvantage in that block distortion is noticeable due to high compression in the same case as in the DCT.
JPEG-2000 has completed as international standards in January 2001. JPEG-2000 uses a system accomplished by combining the wavelet transform with high-efficient entropy coding (bit modeling and arithmetic coding for each bit-plane), thus realizing remarkably improved coding efficiency higher than that of JPEG.
In these international standards, only standards for a decoder are defined. Standards for an encoder can be freely designed. On the other hand, there is no standard regarding means for reducing large loads on processes of the entropy coding and means for effectively controlling a bit-rate in order to realize target compressibility. Therefore, it is important to establish the know-how to reduce the loads and to control a bit-rate. Particularly, in JPEG, it is difficult to control a bit-rate. In many cases, coding has to be performed many times until a target bit-rate is achieved. However, it results in long processing time. In JPEG-2000, it is desired to obtain a target bit-rate by one implementation of coding.
JPEG-2000 is developed specifically for still images. Accordingly, in JPEG-2000, support for interlaced images which often exist in moving images is not studied enough. Therefore, if techniques for still images are applied to moving images, the image quality is noticeably degraded.